Re: [PATCH v2 0/6] sysctl: add typed field descriptors

[Joel Granados]

On Mon, Sep 21, 2026 at 12:54:47PM +0200, Alexey Gladkov wrote:
> Hi,
> 
> Some sysctl users allocate a private copy of an otherwise static ctl_table
> for every namespace or device, then rewrite data and limit pointers before
> registration.  Besides the per-instance allocation, these fixups are often
> addressed by table index, so changing the table can silently associate an
> entry with the wrong data or limits.

In other words the motivation is to:

1. Remove the per-instance allocation thereby reducing the memory
   footprint
2. Remove the possibility of mistakenly associating one variable with a
   different sysctl name (because of un-synced changes)

Only those two. Right?

> 
> This series adds struct sysctl_field as an alternative descriptor for such
> tables.  A field records the value type and an offset into an object
> selected at registration time.  The type-specific offset helpers are small
> wrappers around offsetof() and only add a compile-time check of the
> referenced member type.  The sysctl core derives the handler, size, data
> and limits, and builds a temporary ctl_table when invoking existing
> handler, permission and BPF interfaces.

Building a temporary ctl_table entry just before calling the
proc_handler is the way to go, IMO. But do you need to adjust all those
members (handler, size, data and limits) always? Sometimes everything
stays the same except for the one ".data" member.

Best

PS: Any meat in sashiko's comments?
    https://sashiko.dev/#/patchset/cover.1789987960.git.legion%40kernel.org

> 
> A struct sysctl_context identifies the object shared by the whole
> registration and is copied into the table header.  Subsystems which need
> more than a namespace can embed it as the first member of a larger context
> and select the object to which field offsets apply.
> 
> Existing ctl_table users are unchanged, and subsystems can migrate one
> table at a time.  Converted tables remain static and read-only instead of
> being copied and patched for every instance.
> 
> Compared with the previous RFC [1], this drops the per-field accessor
> functions and the macros which generated them.  The first two patches
> prepare the sysctl core without changing its external behaviour.  The third
> patch adds the new descriptor, followed by conversions of the IPC, mqueue
> and ucount tables as small users of the interface.
> 
> This is only the first part of the conversion.  Network sysctls and the
> other subsystem-specific users will be submitted as separate follow-up
> series so they can be reviewed by their respective maintainers without
> making this initial series excessively large.
> 
> [1] https://lore.kernel.org/all/cover.1787770053.git.legion@xxxxxxxxxx/
> 
> Alexey Gladkov (6):
>   proc: sysctl: address table entries by index
>   sysctl: add unsigned int limit constants
>   sysctl: add typed field descriptors
>   sysctl: ipc: use typed fields for IPC namespace sysctls
>   sysctl: mq: use typed fields for IPC namespace sysctls
>   sysctl: use typed fields for ucount limits
> 
>  fs/proc/inode.c        |   2 +-
>  fs/proc/internal.h     |   2 +-
>  fs/proc/proc_sysctl.c  | 601 ++++++++++++++++++++++++++++++-----------
>  include/linux/sysctl.h | 150 +++++++++-
>  ipc/ipc_sysctl.c       | 188 ++++++-------
>  ipc/mq_sysctl.c        | 104 +++----
>  kernel/sysctl.c        |   3 +
>  kernel/ucount.c        |  64 ++---
>  8 files changed, 750 insertions(+), 364 deletions(-)
> 
> 
> base-commit: 587858367581b9c55c3690f4e63382ad622719d4
> -- 
> 2.55.0
>

Attachment: signature.asc
Description: PGP signature